FIG. 29A is a plan view showing a configuration of an ordinary microstrip line according to a first prior art. FIG. 29B is a longitudinal sectional view taken along a D-D′ line shown in FIG. 29A. FIG. 30 is a perspective view of the microstrip line shown in FIGS. 29A and 29B.
As a method of transmitting a digital signal on a printed circuit board, a method using a microstrip line is normally adopted, in which uses the microstrip line is configured to include a strip conductor 12 and a grounding conductor 11 (FIGS. 29B and 30) with a dielectric substrate 10 sandwiched between the strip conductor 12 and the grounding conductor 11 as shown in FIGS. 29A, 29B and 30. As a transmission line of the microstrip line, various microstrip line-type transmission lines have been known such as a single-ended transmission line, a differential transmission line and a coplanar transmission line. The microstrip line is characterized as follows. If material characteristics of the transmission line and a substrate are uniform, a characteristic impedance is decided by shapes of the transmission line, and the substrate and a signal transmission characteristic having the uniform characteristic impedance can be obtained.
However, if a wiring layout is designed on a printed circuit board using the above-stated microstrip line, it is frequently required to change a line width halfway along the line or to design the microstrip line. In this way, because the shape of the line is discontinuous, the characteristic impedance of the transmission line changes. Furthermore, an amount of this change in the characteristic impedance depends on a frequency. As a result, the change in the characteristic impedance disadvantageously causes deterioration in a waveform of a transmission signal.
As measures against the above-stated waveform deterioration, there has been known a design method for suppressing signal deterioration by minimizing the change in the characteristic impedance as much as possible (See, for example, Patent Document 1).
FIG. 31A is a cross-sectional view of a wiring board that uses a microstrip line according to a second prior art. FIG. 31B is a longitudinal sectional view taken along a line A-A′ shown in FIG. 31A. FIG. 31C is a longitudinal sectional view taken along a line B-B′ shown in FIG. 31A. FIG. 31D is a longitudinal sectional view taken along a line C-C′ shown in FIG. 31A. The microstrip line according to the second prior art is intended to reduce discontinuity of the characteristic impedance according to the prior art described in the Patent Document 1. A method of designing a microstrip line according to the second prior art will be described below with reference to FIGS. 31A to 31D. It is noted that a core material 140 of the wiring board that used the microstrip according to the second prior art is shown.
Referring to FIGS. 31A to 31D, the microstrip is configured to include grounding conductor 110, strip conductor 120 (FIGS. 31B to 31D), a convex portion 121 (FIG. 31D) of the strip conductor 120, and a dielectric material 130 (FIGS. 31B to 31D). Additionally, the microstrip according to the second prior art is configured so that a width of the strip conductor 120 and a distance between the grounding conductor 110 and the strip conductor 120 changes between cross-sections B-B′ and C-C′. Therefore, by changing a capacitance between the grounding conductor 110 and the strip conductor 120, it is advantageously possible to suppress an amount of a change in a characteristic impedance of the transmission line.
FIGS. 32A to 32D and 33 show an example of a configuration of a microstrip line according to a third prior art which has discontinuity and in which a grounding conductor is eliminated halfway. FIG. 32A is a front view of the microstrip line according to the third prior art. FIG. 32B is a plan view of the microstrip line shown in FIG. 32A. FIG. 32C is a longitudinal sectional view taken along a line E-E′ shown in FIG. 32B. FIG. 32D is a side vide of the microstrip line shown in FIG. 32A. FIG. 33 is a perspective view of the microstrip line shown in FIGS. 32A to 32D.
In the case of the microstrip line shown in FIGS. 32A to 32D and 33, a capacitance between a strip conductor 12 and a grounding conductor 11 (FIGS. 32A, 32C, 32D, and 33), in which the strip conductor 12 and the grounding conductor 11 sandwich a dielectric substrate 10, is not present in a portion in which the grounding conductor 11 is not present. Therefore, with the method described in the Patent Document 1, an amount of a change in a characteristic impedance of the microstrip line cannot be reduced as desired and the method produces no advantageous effects.
Moreover, there has been known a design method using a high frequency metamaterial theory (See Non-Patent Document 1) as a design method for controlling characteristics of a transmission line.
FIG. 34 is a circuit diagram showing an equivalent circuit to a transmission line model that illustrates a high frequency material concept that is a design theory disclosed in the Non-Patent Document 1. Referring to FIG. 34, an outline of the high frequency metamaterial design theory will be described.
An equivalent circuit to an ordinary microstrip line can be represented as a ladder circuit configured to include inductors L1 and capacitors C1 shown in FIG. 34. The high frequency metamaterial design theory is the following circuit design method. A microstrip line is realized by adding inductors L1 and L2 and capacitors C1 and C2 to a transmission line having terminals T1, T2, T3, and T4, which leads to development of an electrical characteristic different from that of the transmission lines according to the prior arts and designing a desired characteristic impedance. The Non-Patent Document 1 shows an example of realizing a small-sized microstrip antenna compared to wavelengths in a high frequency electromagnetic field and a unique characteristic impedance corresponding to an effect of a negative index of refraction, and describes a method of controlling a characteristic impedance of a transmission line.    Patent Document 1: Japanese Patent Laid-Open Publication No. JP 2001-053507 A.    Non-Patent Document 1: C. Caloz et al., “Application of the transmission line theory of left-handed (LH) materials to the realization of a microstrip “LH line””, IEEE-APS International Symposium Digest, Vol. 2, pp. 412-415, June 2002.